Current limiting choke coil

ABSTRACT

A device having cores of metal oxide ceramic (for example, Y-Ba-Cu-O) for limiting a short circuit current in power supply systems. The concept provides that a choke core, when operated at a rated current, is superconductive and its shielding currents keep the resulting inductance in the choke at a low level. In the event of an overload, the winding of the choke generates a correspondingly high magnetic field in the core which puts the core into the normally conducting state. This causes the shielding currents to disappear in connection with a rise in the resulting inductance, thus limiting the current. In order to realize a particularly high inductance in the normally conductive case, the superconductive choke core may be made hollow and may be filled at least in part with a ferromagnetic material.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a current limiting choke coil including a coilthrough which current flows and particularly to a current limiting chokecoil with a metal oxide ceramic superconductive core.

Super conducting switching devices are known (U.S. Pat. No. 2,946,030).Such a prior art device includes, in addition to the winding penetratedby alternating currents, a control coil for direct currents having aswitching element actuated by feeding current into the control coil. Ifthe control coil does not carry any direct current, the switchingelement has a very low impedance so that a high current is able to flowin the alternating current circuit. By feeding an appropriate directcurrent into the control winding, the superconductivity of the core isremoved so that the device has a high impedance which reduces thealternating current. Also known are metal oxide ceramic superconductors(IEEE Spectrum, Volume 25, No. 5, May, 1988; K. Fitzgerald,"Superconductivity: Fact vs. Fancy", pages 30-41.

BACKGROUND OF THE RELATED ART

Upon a malfunction, high power energy supply electrical systems aresubjected to extremely high electrodynamic stresses as a result of shortcircuit currents. Although a circuit breaker associated with sections ofa malfunctioning system can interrupt a short circuit current, fullshort circuit current will flow in each case. Hence, expandingelectrical power generation and transmission involves increasing shortcircuit powers resulting in increased electromechanical forces in theoperating media in the event of malfunctions. These increasedelectromechanical forces occur primarily at locations of high powerconcentration and at system coupling points. Often over-sized bus bars,switching devices and transformers are employed in order to accommodatea future increase in short circuit power. Existing system componentsthat are too weak must possibly be reinforced in the course of systemexpansion or replaced by new devices. Costs of expanding the electricalpower capability of the systems involving high power concentration canbe reduced if the short circuit currents can be limited. In athree-phase system this is accomplished with simple air chokes asdescribed in Techn. Mitt. AEG-TELEFUNKEN [Technical News fromAEG-TELEFUNKEN]61 (1971), No. 1, pages 58-63. These chokes exhibit acurrent proportional voltage difference which, although they appear tobe limited in the case of a short circuit, in many cases under normalload takes on values which are too high to maintain stability of systemoperation. More favorable than simple chokes are devices having anon-linear current-voltage characteristic. This includes a limitingcoupler as described in ETZ-A 87 (1966), pages 681-685. This coupleroperates as a series resonant circuit which is tuned to the systemfrequency and, in normal operation, constitutes a very small resistance.A non-linear resistance combination in parallel with the capacitance ofthe resonant circuit takes care that, upon a malfunction, the resonancecondition is cancelled and the inductance limits the current. However,the limiting coupler, developed as a coupling between two high powersystems, has not found acceptance as a short circuit current limiter,primarily because of the high cost of the capacitor battery in theresonant circuit.

The development of superconductors for use at high current densities andwith large magnetic fields has led to numerous solutions and proposalsfor current limiting switching devices. The publication El. Rev. Int.,Vol. 202 (1978) No. 5, pages 63-65, reports of a short circuit currentlimiter having three pairs of transductors whose iron cores aremagnetically saturated in normal operation with the aid of asuperconductive current loop in that they are flooded by a normallydirect field and exhibit a low inductive resistance. However, in thecase of a short circuit, the increased alternating current amplitudecancels out the direct current flowing in the individual transductors byhalf-waves so that each pair of transductors acts as a high inductancechoke.

Other devices utilize the sudden rise in resistance during thetransition from superconductive to normally conductive state. In thecase of an overload, this transition is brought about by exceeding thecritical current density and the critical magnetic field in therespective conductor arrangement.

In Adv. Cryogen. Engng., Vol. 13 (1968), pages 25-50, an arrangement isdescribed which employs a metal conductor path that is cooled withliquid helium and is superconductive at a rated current. With the aid ofa separately excited magnetic field winding, this conductor path isconverted to a normally conductive state as soon as an unduly highcurrent increase is detected in the protecting circuit. The high currentcryotron according to German Patent No. 1,228,701 (1969) operates with asuperconductive gate conductor configuration which loses its capabilityto become superconductive when a current threshold is exceeded andbecomes an ohmic resistance due to the inherent magnetic field of thearrangement, which is possibly supported by extraneous fields. Thearrangement for limiting excess current in electrical power supplysystems disclosed in DE-A 2,712,990 (1977) operates with asuperconductive cable section. In this cable, the normally conductingand the superconducting components are dimensioned, with respect tomaterials ann cross-section, so that, after the critical responsecurrent has been exceeded, a normally conductive current path suddenlyresults, that is, a current path exhibiting a resistance, which limitsthe current.

Such current limiters have not been employed in power supply systems,primarily because of the high cryogenic expenditures for circulating thehelium required to operate metal superconductors at temperatures from 4to 12K. Moreover, their specific resistance in the normally conductivestate is very low at low temperatures.

This applies primarily for high current superconductors stabilized bycopper or aluminum whose specific resistance at operating temperaturelies in an order of magnitude of 10⁻⁸ Ohm cm. Thus, such switchingdevices require long conductor lengths so as to utilize the differencebetween the resistance in the superconductive state and in the normallyconductive state.

SUMMARY OF THE INVENTION

It is an object of the invention to further develop a device of theabove-mentioned type so that, with the simplest possible configurationand economical operation, it can be employed as a protection device inalternating current circuits.

This is accomplished according to the invention in that the portion ofthe core that is capable of superconductivity is composed of a metaloxide ceramic superconductor; the core has only one winding; thealternating current flowing in the winding at the system frequency; andthe threshold of the magnetic field is generated n the winding by athreshold current. With this device it is possible to considerablyreduce cryogenic expenditures and material costs.

Oxide ceramic superconductors have transition temperatures in a range of90K and have a specific resistance which is several orders of magnitudehigher once the superconductive state no longer exists, than theresistance of extensively cooled metal conductors. Due to the increasein the ohmic resistance of the core beginning at a given currentthreshold, the current is forced to flow through the high maininductance if the currents are small in the core.In this way, currentgenerated, for example, by a short circuit in a power distributionsystem, is limited.

The current limiting choke as a whole, or at least its core, is cooledby liquid nitrogen. Cooling with liquid nitrogen is sufficient to keepthe core at the temperature required for superconductivity.

In a suitable embodiment, the core has a toroidal shape around which thewindings are placed in the form of an annular coil. This configurationinvolves low stray losses.

It is particularly favorable to configure the superconductive hollowbody alternatingly of superconductive and ferromagnetic elements.inductance of the choke can be increased considerably in that thesuperconductive hollow body is filled completely or in part with aferromagnetic material. It is also advisable to construct the choke corealternatingly of elements capable of superconductivity and offerromagnetic elements.

The use of ferromagnetic material in conjunction with thesuperconductive core considerably augments the magnetic flux so that thecurrent limiting effect of the choke in the case of a short circuit isimproved. On the other hand, the dimensions of the choke coil can bereduced while retaining the inductance determined for a specific case.

Preferably, the susceptibility of the ferromagnetic material below thecritical temperature of the superconductive material of the core and ofthe elements, respectively, has a high value which is typical forferromagnetic substances. The core of the choke coil is cooled to suchan extent that the oxide ceramic superconductor has a temperature whichis lower than its transition temperature, for example 90 K. Theferromagnetic material must have a high susceptibility at a temperaturewhich lies below the transition temperature.

In a suitable embodiment, the ferromagnetic material is thermallyinsulated from the superconductive core and the superconductiveelements, respectively, so that it can be held at a temperature at whichthe susceptibility has a high value typical for ferromagneticsubstances. In this embodiment, it is not necessary to employ aferromagnetic material that retains a high susceptibility at lowtemperatures. In particular, the temperature can be regulated to a valuewhich lies lower than room temperature and at which there still existssufficiently high susceptibility.

In an advantageous embodiment, a ferromagnetic body is provided with alayer of a metal oxide ceramic superconductor. Such a core configurationis very simple. The ferromagnetic body must retain its susceptibility atlow temperatures. If a ferromagnetic material is employed which does nothave a high susceptibility at low temperatures, then preferably thermalinsulating layer is provided on the ferromagnetic body, with a layer ofa metal oxide ceramic superconductor being disposed on the insulatinglayer.

In a particularly favorable embodiment, the superconductive core and thesuperconductive elements are composed of individual juxtaposed segmentsof metal oxide ceramic material. With such a configuration it ispossible to realize a large core structure.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described in greater detail with reference toan embodiment thereof that is illustrated in the drawings, which willreveal further details, features and advantages.

It is shown in:

FIG. 1, an alternating current circuit including a short-circuit currentsensor device;

FIG. 2, a sectional view of a cylindrical choke coil according to theinvention;

FIG. 3A, a top view of a choke coil having a toroidal core and anannular winding;

FIG. 3B, a cross sectional view of the toroidal core of FIG. 3A;

FIG. 4, a special version of a choke core that is able to becomesuperconductive in the form of a hollow cylinder having end pieces atits frontal faces;

FIG. 5, a special version of the superconductive choke core in the formof a hollow cylinder containing ferromagnetic material;

FIG. 6, a choke core with a thermally insulating layer 17;

FIG. 7, a diagram of the dependency of the quotient of the inductance ofthe choke coil and the inductance at rated current upon the quotient ofthe current through the choke coil and the rated current;

FIG. 8, a cross-sectional view of an additional embodiment of a chokecoil;

FIG. 9, a sectional view of a choke coil composed of alternatingelements of superconductive and of ferromagnetic material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, the numeral 1 identifies an alternating current source(generator, transformer), the numeral 2 a load, 3 a current limitingchoke coil including a superconductive core 4 and an inductance L. Thecore 4 is made superconductive by cooling it to below the transitiontemperature of the core material and serves to keep the inductance ofchoke 3, in view of the shielding currents in the core, at the low valueL=L₁. A voltage drop of ΔU=I₁ ψL₁ occurs across choke 3, with I₁identifying the current flowing during operation, and ψ the radianfrequency of the system. The voltage across load 2 is here assumed tohave the value U and the voltage of current source 1 has the value U+ΔU.A short circuit, indicated by an arrow in FIG. 1, signifies that theimpedance of the load and its operating voltage U go toward zero.Without special measures, the short circuit current flowing then wouldbe I₁ =(U+ΔU)/(ψL₁). This is prevented, according to the invention, inthat, if there is an undesirable rise in current, the superconductivityin the core of choke coil 3 is cancelled out by the critical currentdensity and the magnetic flux density being exceeded. Consequently, theshielding currents disappear and the magnetic flux is able to fullypenetrate the interior of the choke coil, resulting in an increase inthe inductance to the value L₂ >L₁. Instead of a short circuit current,the current now flowing is I₂ =(U+ΔU)/(ψL₂). The impedances of thecurrent source and of the lines are neglected in this consideration.

For explanation, a numerical example including the following data shallbe considered: U=63.6 kV, I₁ =2 kA, ψ=314 s⁻¹, L₃ =3 mH, ΔU=1.9 kV.These numerical values result in a short circuit current I_(K) =69.5kA≈35 I₁.

If in the case of a malfunction, the current is limited, for example, toI_(K) /5≈14 kA, the forces in the current carrying operating media dropto 1/25 of the forces due to the electrodynamic stress caused by thefull short circuit current. In order to meet the condition of I₂ =I_(K)/5, the current limiting inductance in the case of a malfunction wouldhave to take on the value

    L.sub.2 =(U+ΔU)/(ψI.sub.2)                       (1)

that is, it would have to rise to 5 L₁. Inductance changes of this typecan be realized with a cylindrical choke coil according to FIG. 2. InFIG. 2, the numeral 5 identifies a metal oxide ceramic core,particularly Y-Ba-Cu-O, which is capable of superconductivity and has adiameter D_(K), the numeral 6 identifies a winding having an averagewinding diameter D₀, a wire thickness d, a height h and a number ofwindings w For d<<D₀, the inductance of the cylindrical coil alone is L₀=D₀ ·Q w² /2, where Q is a geometry factor listed in a table byKohlrausch in Praktische Physik [Practical Physics ], Volume 2, (1944),page 204, as a function of D₀ /h.

For a cylindrical winding with core, the following considerationapplies: if the temperature of the core material falls below itstransition temperature T_(c), the core becomes superconductive and urgesthe magnetic flux into the annular chamber between core and winding. Inorder to approximately calculate the resulting inductance in this state,the core can be replaced by a concentric second cylindrical winding ofthe same height having a diameter D_(K) and being wound in the oppositedirection. The inductance of this equivalent circuit is:

    L.sub.1 =(D.sub.0 ·Q·(D.sub.0 /h)-D.sub.K ·Q·(D.sub.K 7h)) w.sup.2 /2             (2)

If the superconductivity in the core is cancelled out because thecritical current density and the magnetic flux density are exceeded, theinductance rises to the value L₀ =L₂ and limits the current to theamount (U+ΔU)/(ψL₂). This simple relationship applies if the specificresistance of the core material is so high that eddy currents induced inthe core without superconductivity have practically no influence on theinductance. The above current limiting concept can be transferred, inprinciple, to windings and cores having different geometries, thus alsoto the toroidal arrangements of FIGS. 3A and 3B which operate withoutinterfering stray magnetic fields. In FIGS. 3A and 3B, the numeral 7identifies the toroidal core on a superconductive ceramic and thenumeral 8 the annular winding surrounding it.

Another version of the core is shown in FIG. 4 for the example of acylindrical choke coil. The core 9 is configured as a hollow cylinderhaving a wall thickness d_(h) and is arranged concentric with winding10.

End pieces 11 and 12 close off the frontal faces of the hollow cylinder.They have the effect that in the superconductive state, the magneticflux of the coil does not penetrate into the interior of the cylinderand thus produces a shielding comparable to that obtained with a solidcylindrical core.

In FIG. 5 the superconductive core 13 is configured as a closed hollowcylinder in which a ferromagnetic material 14 is disposed. Core 13 issurrounded by a winding 15.

The ferromagnetic material must retain its susceptibility at lowtemperatures. A ferromagnetic material is employed which at highertemperatures, for example at room temperature, has a high susceptibilitywhich remains in effect in a range of 90K.

In the embodiment shown in FIG. 6, a cylindrical body 16 offerromagnetic material is surrounded by a layer 17 of thermal insulatingmaterial. Layer 17 is in turn surrounded by a hollow cylindricalsuperconductive core 18 which has a cylindrical winding 19 arranged onits exterior face. Layer 17 insulates body 16 from core 18. Moreover,body 16 is connected, for example by way of a base 20, with othercomponents whose temperature is higher than the transition temperatureof core 18. Therefore body 16 has a higher temperature than core 18 andmay be composed of ferromagnetic material which at low temperatures inthe range of the transition temperatures of core 18 loses its highsusceptibility typical for ferromagnetic substances.

In order to produce a flat magnetization characteristic, ferromagneticbodies 14 and 16 may also be configured as a closed circle alternatinglycomprising sections of material capable of superconductivity and offerromagnetic material. The hollow cylindrical configuration may then beomitted.

Or, the superconductive core may be applied as a layer to aferromagnetic, for example, cylindrical or toroidal body. If the bodyretains its high susceptibility even at low temperatures, core and bodymay be connected directly with one another. Such an arrangement has theadvantage that the core and the body can be cooled together. Often thissimplifies the structural arrangement for the cooling. This applies todevices in which the ferromagnetic material retains its susceptibilityin the range of the transition temperature of the core. If thesusceptibility drops to undesirably low values in the range of thetransition temperature, then a thermal insulating layer must be providedbetween the ferromagnetic body and the core, onto which the core, inparticular, can be applied as a layer.

If the temperature of the core material falls below its transitiontemperature T_(c), the core becomes superconductive and urges themagnetic flux into the annular space between core and winding. The chokecoil therefore has a low inductance.

If the superconductivity in the core is cancelled out by the criticalcurrent density and the magnetic flux density being exceeded, theinductance increases considerably. At the system frequency, theabove-described choke coil may have a low impedance compared to the loadimpedance.

The choke impedance ψL₁ at rated current I₁, for example, has thefollowing relationship to the load impedance Z:

    ψL.sub.1 =p·Z                                 (3)

where p may equal 0.01. In the case of a short circuit, there remainsthe residual impedance:

    Z.sub.K =q·Z                                      (4)

In the current limitation considerations below, a calculation withcomplex resistances is omitted for the sake of simplicity since p aswell as q<<1.

Under the mentioned conditions, the following applies for the ratedcurrent if the choke core is superconductive

    I.sub.1 =U/(Z+ψL.sub.1)=U/(1+p)Z                       (t)

If current I rises, the superconductivity in the ceramic core is loststarting at a certain threshold. With increasing magnetic field, analmost steady increase of normally conductive regions is observed in thevolume of an oxidic superconductor having a high transition temperature.Consequently, the inductance is a function of the current I. For thefurther considerations below, it is approximated in the following form:

    L/L.sub.1 =a(I/I.sub.1 -1)+1                               (6)

where the coefficient marked a must be determined from measurements.Using the abbreviation x=I/I₁, the following relationship can bederived:

    I(p(a(x-1)+1)+q)=U/Z=I.sub.1 (1+p)                         (7)

where U identifies the system voltage.

This leads to the following equation:

    x.sup.2 +((p-pa+q)/(pa))x-(1+p)/(pa) =0                    (8)

from which the relationship between short circuit current and ratedcurrent can be calculated if a, p and q are known.

EXAMPLE

FIG. 7 shows the evaluation of an experiment for the determination ofthe coefficient a. Measured was the increase in the inductance of achoke coil in a magnetic field. The superconductive core was a hollowceramic cylinder having an exterior diameter of 20 mm, an interiordiameter of 16 mm and a height of 30 nun. The winding had 80 turns, alength of 26 mm, an average diameter of 21 mm. With the coresuperconductive, the inductance was L₁ =μH, with a completely normallyconductive core, it was L₀ =83 μH, measured at a frequency of 10 kHz.For L/L₁ as a function of I/I₁, an S-shaped curve resulted which had anaverage slope a=0.41.

The effect of an analog choke as a current limiter will now be discussedwith reference to an example. For the system parameters according toEquations (3) and (4) the following numerical values are assumed toexist: p=0.01 and q=0.03.

With a=0.41 (according to FIG. 7), Equation (8) furnishes the currentratio x=11.9. In the case of a short circuit, the current under theseconditions would be limited to roughly twelve times the value of therated current. The unlimited short circuit current, calculated for thesame parameters, would reach 25 times the rated current.

A farther reaching limitation of the short circuit current can berealized with an L(I) choke coil characteristic that is steeper thanshown in FIG. 7. With a slope of a=2.6, also shown in FIG. 7, and againwith p=0.01 and q=0.03, the short circuit current could be limited tosix times the rated current.

With large core dimensions which cannot be produced by conventionalmanufacturing methods for high transition temperature superconductors,the superconductive cores are subdivided as shown in FIG. 8 for part ofa core 21. Core 21 is composed of individual superconductor segments ofwhich FIG. 8 identifies superconductor segments 22, 23, 24, 25, 26 and27. Superconductor segments 22, 23 and 24 are disposed in a radiallyoutward position on core 21 while superconductor segments 25, 26 and 27take up a radially inward position. More than the two layers shown inFIG. 6 may also be provided. Core 21 therefore has a polygonal crosssection. Superconductor segments 22 to 27 form parts of the polygon.Core 21 is surrounded by a winding 28. A shielding current generallymarked 29 flows in each one of superconductor segments 22 to 27 of thecore and displaces the magnetic flux as a whole from the core region inthe same manner as a corresponding ring current flows along theperiphery. The thickness of the "grooves" between the core portions ishere selected to be small compared to the core diameter.

The choke coil according to FIG. 8 may advisably have a cavity offerromagnetic material. However, it also operates without ferromagneticmaterial, for example, as a solid core. It may be designed for highrated currents.

FIG. 9 shows a choke coil 30 including a core 31 composed of alternatingelements 32 and 33 of superconductive and ferromagnetic material. It ishere assumed that the ferromagnetic material has a sufficiently highsusceptibility even below the transition temperature of superconductiveelements 32. Should this not be the case, a thermal insulation must beprovided between elements 32 and 33.

Choke coil 30 has a yoke 34 of ferromagnetic material.

With respect to dimensioning and engineering development of a currentlimiting choke according to the invention, it may be advisable tooperate the core material shortly below its transition temperature so asto keep the requirement for magnetization low for a transition fromsuperconductivity to normal conductivity.

As soon as the limit current has cancelled out the superconductivity inthe core, induction within the core temporarily produces heat, with thepower density being a function of the specific resistance of thenormally conducting core material and of the current in the choke. Thethermal inertia of the core prevents it from dropping back into thesuperconductive state before the power switch associated with themalfunctioning system section has opened the short circuited connection.The time required to do this customarily is 1 to 2 periods of the systemfrequency.

Advisably the choke is cooled as a whole. Doing this, a very closemagnetic coupling is possible between core and winding without cryogenicseparation as it would be required only if the core were cooled. On theother hand, the ohmic losses in the winding are low since, at the liquidnitrogen temperature, the specific resistance of the conductor materialof the winding drops to roughly 1/10 of its value at room temperature.

We claim:
 1. A device comprising;a single winding; and a coil corearranged in said single winding, said coil core comprisingsuperconductive components of metal oxide ceramic superconductivematerial and ferromagnetic components of ferromagnetic material, whereinif the amplitude of an alternating current applied to said singlewinding exceeds a threshold value at a system frequency, said singlewinding generates a threshold magnetic field which converts saidsuperconductive components into non-superconductive components.
 2. Adevice according to claim 1, further comprising:cooling means forcooling at least said coil core with liquid nitrogen.
 3. A deviceaccording to claim 1, whereinsaid coil core has a toroidal shape andsaid single winding has an annular shape.
 4. A device according to claim1, whereinsaid coil core is hollow and is composed alternatingly ofelements capable of superconductivity and of ferromagnetic elements. 5.A device according to claim 4 whereinat temperatures below the criticaltemperature of the superconductive elements, the susceptibility of saidferromagnetic elements is typical of the susceptibility of ferromagneticsubstances at room temperatures.
 6. A device according to claim 4,whereinsaid ferromagnetic elements are thermally insulated from saidsuperconductive elements and are held at a higher temperature than thetemperature of said superconductive elements.
 7. A device according toclaim 1, whereinsaid superconductive elements are composed of individualsuperconductive segments which border on one another.
 8. A deviceaccording to claim 7, wherein the superconductive components and theferromagnetic components are alternately arranged.
 9. A devicecomprising:a single winding; and a core arranged in said single winding,said core formed from a metal oxide ceramic superconductive component,wherein if the amplitude of an alternating current applied to saidsingle winding exceeds a threshold value at a system frequency, saidsingle winding generates a threshold magnetic field which converts saidsuperconductive component into a non-superconductive component.
 10. Adevice according to claim 9, further comprising cooling means forcooling at least said core with liquid nitrogen.
 11. A device accordingto claim 9, wherein the core is configured as a solid cylinder
 12. Adevice according to claim 9, wherein the core is configured as a toroid.13. A device for changing an inductance of a choke comprising;a singlewinding; and a core arranged in the single winding for producing a firstchoke inductance when an alternating current applied to the singlewinding is less than a threshold at a predetermined frequency, and forproducing a second choke inductance when the amplitude of an alternatingcurrent exceeds the threshold, the core formed from a metal oxideceramic superconductor material so that a magnetic field produced by thecurrent converts the superconductive material into a normally conductivematerial when the amplitude of the current exceeds the threshold. 14.The device for changing an inductance of a choke according to claim 13further comprising a cooling device for cooling the core with liquidnitrogen.
 15. The device for changing an inductance of a choke accordingto claim 13 wherein the core is further formed from alternating elementsof the superconductive material and a ferromagnetic material.
 16. Thedevice for changing an inductance of a choke according to claim 15,further comprising a thermal insulator between the superconductiveelements and the ferromagnetic elements.
 17. The device for changing aninductance of a choke according to claim 13, wherein the core is formedfrom a plurality of segments of the superconductive material.
 18. Thedevice for changing an inductance of a choke according to claim 13,wherein the core is configured as a solid cylinder.
 19. A devicecomprising;a winding; and a coil core arranged in the winding, the coilcore being hollow and comprising alternatingly arranged superconductivecomponents of metal oxide ceramic superconductive material andferromagnetic components of ferromagnetic material, at temperaturesbelow the critical temperature of the superconductive elements, thesusceptibility of the ferromagnetic elements being typical of thesusceptibility of ferromagnetic substances at room temperatures, whereinif the amplitude of an alternating current applied to the windingexceeds a threshold value at a system frequency, the winding generates athreshold magnetic field which converts the superconductive componentsinto non-superconductive components.
 20. A device comprising:a winding;and a coil core arranged in the winding, the coil core being hollow andcomprising alternatingly arranged superconductive components of metaloxide ceramic superconductive material and ferromagnetic components offerromagnetic material, the ferromagnetic elements being thermallyinsulated form the superconductive elements and held at a highertemperature than the temperature of the superconductive elements, attemperatures below the critical temperature of the superconductiveelements, the susceptibility of the ferromagnetic elements being typicalof the susceptibility of ferromagnetic substances at room temperatures,and wherein if the amplitude of an alternating current applied to thewinding exceeds a threshold value at a system frequency, the windinggenerates a threshold magnetic field which converts the superconductivecomponents into non-superconductive components.
 21. A device forchanging an inductance of a choke comprising:a winding; and a corearranged in the winding for producing a first choke inductance when analternating current applied to the winding is less than a threshold at apredetermined frequency, and for producing a second choke inductancewhen the amplitude of an alternating current exceeds the threshold, thecore formed from alternating elements of a metal oxide ceramicsuperconductor material and a ferromagnetic material with a thermalinsulator between the superconductive elements and the ferromagneticelements, a magnetic field produced by the current converts thesuperconductive material into a normally conductive material when theamplitude of the current exceeds the threshold.